4.1.1 Sources of Greenhouse Gases

Substantial, pre-industrial abundances for CH4 and N2O
are found in the tiny bubbles of ancient air trapped in ice cores. Both gases
have large, natural emission rates, which have varied over past climatic changes
but have sustained a stable atmospheric abundance for the centuries prior to
the Industrial Revolution (see Figures 4.1 and 4.2).
Emissions of CH4 and N2O due to human activities are also
substantial and have caused large relative increases in their respective burdens
over the last century. The atmospheric burdens of CH4 and N2O
over the next century will likely be driven by changes in both anthropogenic
and natural sources. A second class of greenhouse gases - the synthetic
HFCs, PFCs, SF6, CFCs, and halons - did not exist in the atmosphere
before the 20th century (Butler et al., 1999). CF4, a PFC, is detected
in ice cores and appears to have an extremely small natural source (Harnisch
and Eisenhauer, 1998). The current burdens of these latter gases are derived
from atmospheric observations and represent accumulations of past anthropogenic
releases; their future burdens depend almost solely on industrial production
and release to the atmosphere. Stratospheric H2O could increase,
driven by in situ sources, such as the oxidation of CH4 and exhaust
from aviation, or by a changing climate.

Tropospheric O3 is both generated and destroyed by photochemistry
within the atmosphere. Its in situ sources are expected to have grown
with the increasing industrial emissions of its precursors: CH4,
NOx, CO and VOC. In addition, there is substantial transport of ozone
from the stratosphere to the troposphere (see also Section
4.2.4). The effects of stratospheric O3 depletion over the past
three decades and the projections of its recovery, following cessation of emissions
of the Montreal Protocol gases, was recently assessed (WMO, 1999).

The current global emissions, mean abundances, and trends of the gases mentioned
above are summarised in Table 4.1a. Table
4.1b lists additional synthetic greenhouse gases without established atmospheric
abundances. For the Montreal Protocol gases, political regulation has led to
a phase-out of emissions that has slowed their atmospheric increases, or turned
them into decreases, such as for CFC-11. For other greenhouse gases, the anthropogenic
emissions are projected to increase or remain high in the absence of climate-policy
regulations. Projections of future emissions for this assessment, i.e., the
IPCC Special Report on Emission Scenarios (SRES) (Nakic´enovic´
et al., 2000) anticipate future development of industries and agriculture that
represent major sources of greenhouse gases in the absence of climate-policy
regulations. The first draft of this chapter and many of the climate studies
in this report used the greenhouse gas concentrations derived from the SRES
preliminary marker scenarios (i.e., the SRES database as of January 1999 and
labelled p' here). The scenario IS92a has been carried along in many
tables to provide a reference of the changes since the SAR. The projections
of greenhouse gases and aerosols for the six new SRES marker/illustrative scenarios
are discussed here and tabulated in Appendix II.

An important policy issue is the complete impact of different industrial or
agricultural sectors on climate. This requires aggregation of the SRES scenarios
by sector (e.g., transportation) or sub-sector (e.g., aviation; Penner et
al., 1999), including not only emissions but also changes in land use or
natural ecosystems. Due to chemical coupling, correlated emissions can have
synergistic effects; for instance NOx and CO from transportation
produce regional O3 increases. Thus a given sector may act through
several channels on the future trends of greenhouse gases. In this chapter we
will evaluate the data available on this subject in the current literature and
in the SRES scenarios.

a All abundances are
tropospheric molar mixing ratios in ppt (10 -12 )and trends are
in ppt/yr unless superseded by units on line (ppb = 10 -9 , ppm
= 10 -6 ). Where possible, the 1998 values are global, annual
averages and the trends are calculated for 1996 to 1998.b GWPs are from Chapter 6 of this
report and refer to the 100-year horizon values. c Species with chemical feedbacks that change the duration
of the atmospheric response; global mean atmospheric lifetime (LT) is given
first followed by perturbation lifetime (PT). Values are taken from the
SAR (Prather et al., 1995; Schimel et al., 1996) updated with WMO98
(Kurylo and Rodriguez, 1999; Prinn and Zander, 1999) and new OH-scaling,
see text. Uncertainties in lifetimes have not changed substantially since
the SAR. d CO trend is very sensitive to the time period chosen.
The value listed for 1996 to 1998, +6 ppb/yr, is driven by a large increase
during 1998. For the period 1991 to 1999, the CO trend was -0.6 ppb/yr.
CO is an indirect greenhouse gas: for comparison with CH4 see
this chapter; for GWP, see Chapter 6.